This invention relates to a method and an apparatus for heat sealing two thermoplastic parts together. More particularly, the invention relates to the heat sealing of thermoplastic lids to thermoplastic containers.
The use of plastic containers for packaging food and other items has become more prevalent as new advances in this field have allowed the production of better containers. Such containers are often formed without an integral lid but rather use an aluminum foil lid which may be relatively easily removed by the consumer. It is common to seal such lids onto the containers by heat sealing. An adhesive, which commonly is a thermoplastic material, is applied to the outer portion of the lid or to the edge or flange of the container or both, the lid is placed on the open end of the container and a heated clamp is lowered onto the lid to press it firmly down on the container and melt the adhesive to form a good heat seal. The use of the hot clamp has the disadvantage that it will not only heat the adhesive but will also heat the material which is within the container thereby releasing steam and increasing the pressure inside the container. It is desirable to eliminate this internal pressure, particularly when the lid has a peelable pull tab because the internal pressure may cause failure of the pull tab. The internal pressure also can cause the destruction of the integrity of the heat seal and the foil lid may budge and/or wrinkle when the hot clamp is applied thereto.
There is a great desire in the industry to use lids made of thermoplastic materials so that foil can be eliminated. The hot clamp method of heat sealing can be used for heat sealing a thermoplastic lid to a thermoplastic container but this method suffers from several disadvantages other than those discussed above. Since the heat is applied from only one side of the construction to be sealed, i.e. from the top of the lid, the heat must travel through the lid material before it can melt the thermoplastic material of the edge or flange of the container. This means that the heat transfer process takes longer than for foil lids, thereby slowing down the processing time, and may also cause undesirable distortion or melting of the lid itself. Additionally, the lid has to be perfectly balanced on the flange of the container in order to get a good seal because otherwise the contact with the hot clamp will be uneven, causing the heat seal to be inconsistent.